1. Field of the Invention
The present invention relates to a vehicular alternator, and more particularly to a vehicular alternator including a permanent magnet for auxiliary excitation.
2. Description of the Related Art
A vehicular alternator usually comprises a rotor and a stator. In general, the rotor comprises a pair of claw-type magnetic poles arranged in an opposed relation, and field windings coiled radially inward of a plurality of claws provided in each of the claw-type magnetic poles. When a current is supplied to the field windings, the pair of claw-type magnetic poles are magnetized to N and S poles. Also, the stator is constituted by coiling stator windings, from which an AC induced voltage is outputted, over a stator core. The rotor is rotated relative to the stator.
With such a structure, a magnetic circuit is formed in which the magnetic flux outgoing from the claw-type magnetic pole magnetized to an N pole returns to the claw-type magnetic pole magnetized to an S pole through the stator windings. The magnetic flux of the magnetic circuit is in an interlinkage relation to the stator windings of the stator, while the rotor rotates relative to the stator. As a result, an AC induced voltage is generated in the stator windings.
In the vehicular alternator having the above-described structure, efforts to reduce the magnetic flux leaked between the claw-type magnetic poles and to intensify the magnetic flux generated from the field windings for an increase in output of the alternator have been made, for example, by interposing a permanent magnet for auxiliary excitation between the claw-type magnetic poles. JP,A 11-318064, for example, discloses such a vehicular alternator including a permanent magnet for auxiliary excitation disposed between the claw-type magnetic poles.
The above-mentioned related are, however, has the following problem.
Usually, claws of a claw-type magnetic pole are each formed into a substantially triangular shape in section taken along the axial direction of a rotor such that the claw is tapered toward its tip, for the purpose of reducing the weight of a claw end portion. This is because the claws of the claw-type magnetic pole are cantilevered. In other words, such a sectional shape is intended to prevent the claw end portion from rising radially outward of the rotor due to centrifugal forces when the rotor rotates at high speeds. Therefore, when a permanent magnet having a relatively large thickness in the radial direction of the rotor is disposed between the adjacent claws of the claw-type magnetic poles, a part of each lateral surface (i.e., magnetic pole surface) of the permanent magnet in the circumferential direction of the rotor does not contact the claw, thus resulting in increased resistance against a flow of magnetic flux (i.e., greater magnetic loss) in a magnetic circuit formed by the permanent magnet. Accordingly, the magnetic flux of the permanent magnet is not effectively utilized.
In the above-described related art, each claw is likewise formed into a substantially triangular shape in section taken along the axial direction of the rotor such that the claw is tapered toward its tip. Although the related art employs a permanent magnet having a relatively small thickness in the radial direction of the rotor, a particular consideration is not focused on the shape of a contact surface between the claw and the permanent magnet. Hence, a part of a magnetic pole surface of the permanent magnet not contacting the claw also occurs near a claw end portion, and effective utilization of the magnetic flux of the permanent magnet is not ensured.